Projections indicate that for yields to keep pace with the expected increase in demand, the application of agrochemicals must be increased with the resultant detrimental impact on the environment, including chemical pollution and aquatic and marine eutrophication. Tilman, David (1999). Global environmental impacts of agricultural expansion: The need for sustainable and efficient practices. Proceedings of the National Academy of Sciences of the United States of America. 96(11): 5995-6000.). For example, it is predicted that another three-fold increase in the rate of nitrogen fertilizer application is necessary to sustain the next doubling of global food production (Tilman, 1999). However, it is estimated that current fertilization practices result in less than ½ of the applied nitrogen either being retained in the field or being taken up by the target crop.
Even though it is abundant in the rhizosphere, the bioavailability of iron is limited by its tendency to form insoluble oxyhydroxide polymers, a situation aggravated in alkaline soils, which constitute ˜30% of the world's arable soils (Guerinot, 2001). Consequently, iron is the third most rate limiting nutrient under field conditions, next to nitrogen and phosphorus (Guerinot, Mary Lou (2001). Improving rice yields—ironing out the details. Nature Biotechnology 19: 417-418.). Iron is vital for normal plant growth and development (Thoiron, S., Pascal, N., and Briat, J-F. (1997). Impact of iron deficiency and iron re-supply during the early stages of vegetative development in maize (Zea mays, L.). Plant Cell and the Environment. 20: 1051-1060) where it is necessary for functions such as oxygen transport and storage, electron transfer (redox reactions), and nitrogen fixation. Iron deficiencies manifest themselves in leaf yellowing and necrosis, poor growth, and general weakness.
Grasses, including the cereal grains, represent the world's most economically important plants. They provide more than ⅔rds the nutrition in human diets worldwide (Cassman, 1999) and occupy almost 40% of global cropland (Tilman, 1999). In contrast to dicots and non-graminaceous monocots, most grasses have evolved a method of sequestering and transporting iron, designated Strategy II (Marschner, H. and V. Rhomsfeld. (1994) Strategies of plants for the acquisition of iron Plant and Soil. 165:261-274), which includes the synthesis and secretion of low molecular weight molecules (phytosiderophores) that chelate iron and move it to the root where the entire complex is taken in through a transmembrane porter.
Like plants, bacteria secrete iron chelating molecules (siderophores). Siderophores and their analogs have tremendous therapeutic potential. One antimicrobial application has involved the attachment of drugs or other biologically relevant molecules to bacterial siderophores, thus providing species-selective conjugates that are actively transported into microbes. Although a number of studies have demonstrated the feasibility of s species-selective siderophore-mediated drug transport in microbial systems, no work has been done with grasses and their corresponding phytosiderophores. If phytosiderophore conjugates are recognized and transported in plants in a manner analogous to bacteria, they would provide a means of targeting effector molecules to a specific plant group or species. Additionally, although the feasibility of heterologous expression of functional phytosiderophore/iron transporter in yeast has been shown (Murata, 2006, cited below), to date no one has demonstrated PS/Fe+3 transporter expression in dicotyledonous plants or other non-graminaceous species. Phytosiderophore transporters expressed in leaves or other aerial tissue of engineered plants, both graminaceous and non-graminaceous, would provide a convenient portal through which to deliver a Fe+3-phytosiderophore-effector molecule complexes to a target while excluding neighboring competitors, thus reducing application rates and runoff.